The present invention relates to an inductive or capacitive proximity detector that is particularly capable of detecting ferrous or non-ferrous metals, or a capacitive proximity detector comprising an oscillator capable of transmitting a measurement signal the frequency of which is a function of the proximity of an object to be detected, and a digital processing circuit that controls an output of the detector as a function of the frequency of the measurement oscillator.
This type of proximity detector is described in German patent DE 32 04 405. In this detector two oscillators are provided that oscillate at the same frequency, one of the oscillators being inducted and the other oscillator not being inducted by the object to be detected. The oscillator outputs are applied to respective counters such that the counting difference observed in the presence of a target is used by a logic circuit that commutes an output signal.
British patent GB 1 510 034 describes an inductive proximity detector comprising a variable frequency oscillator and a reference oscillator that operate with a detection counter and a reference counter, said reference counter defining a counting interval for the detection counter.
The known proximity detectors do not take into account variations or drifts affecting the components of the processing circuit, particularly depending on the temperature, which therefore results in insufficient measurement accuracy and reliability.
The aim of the invention is to detect objects, particularly ferrous or non-ferrous metal objects, using frequency processing that ensures excellent measurement accuracy.
According to the invention the set point counter value is stored in a memory associated with the digital processing circuit and it is correctable particularly according to the temperature, and a calibration counter value is introduced into the memory when the detector is calibrated in the presence of an object located at the nominal range, the set point counter value being obtained by correcting the calibration value according to the temperature.
Preferably, the set point counter value is corrected cyclically, for example at each measurement or every n measurements, n being a predetermined number, according to the ambient temperature measured at the time.